x2.i am paleo but i make my living teaching people how to identify and eat wild plants. A lot of vegetarians sign up for my classes and i hear this view often. I think it includes many assumptions. Let me address just one. Vegetarians assume more plants grown for food is better than more animals being raised for food. It goes along the lines of if we take a 100,000 acres out of beef production and put it into plant production we can feed a more people. That's assertion needs one more word: We can feed more people poorly. Increasing carb production never means people eat better. It never means more people eat better. It always means more people will eat poorly. Producing more carbs does not translate into healthier people or more people fed better. It just means more people eating poorly. That does not seem to me to be positive or "sustainable."
of course the real problem is too many people. Increasing the food supply is the bandaid on the infected wound. We have gmos and agri-business producing huge amounts of grain because there are so many people. Vegetarians like to paint meat eaters and their food animals as the problem. It is not: It is too many people and we have that because of grain production. More grain production only means more people. That is not a solution. It's making the problem worse. Getting rid of food animals is not a solution. That means less healthy people. Population reduction is the solution. Humanity however is not yet committed or desparate enough to use the solution.